1. Field of the Invention
The present invention relates to displays, and in particular relates to drive circuits of the displays.
2. Description of the Related Art
FIG. 1 is a schematic diagram illustrating the driver circuit in the prior art. The drive circuit 100 comprises a pixel 102 and an output stage 104 used for driving the pixel 102. The output stage 104 of the driver circuit 100 further comprises a p-type MOSFET (PMOS) 112 and an n-type MOSFET (NMOS) 114, and each of the transistors 112 and 114 comprises a gate coupled to a pixel signal Sp and controlled by the pixel signal Sp to switch an output voltage Vout on the pixel 102 between a high level VH and a low level VGND.
The output voltage Vout on the pixel 102 influences the brightness of the pixel, while the characteristic of the display influences that as well. Taking the carbon nanotube display (CNDP) for example, owing to its particular characteristic, the brightness of the CNDP will increase when it ages. For this case, it is necessary for the drive circuit 100 to comprise a calibration device 130 to calibrate the brightness of the display. For example, in the calibration device 130 in FIG. 1, the transmission gate composed of a PMOS T1 and a NMOS T2 could be controlled by a bias voltage Vbias to calibrate the equivalent resistance of the calibration device 130 to further adjust the brightness of the pixel 102.
However, the coupling effect of the transistor T1 (there is a coupling capacitor between the gate and the source/drain) makes the output voltage Vout influencing the bias voltage Vbias. The output voltage Vout on the pixel 102 alternates between the two voltage levels according to the pixel signal SP. When the output voltage Vout switches from the low voltage VGND to the high voltage VH, the output voltage makes the bias voltage Vbias raise rapidly and causes a surge P1 therein; when the output voltage Vout switches from the high voltage VH to the low voltage VGND, the output voltage makes the bias voltage Vbias descend rapidly and causes a surge P2 therein. In addition, the drive circuit 100 of the display is a high voltage device, and the high voltage VH on the pixel 102, for example, could be as high as 110 volt, therefore, the surge P1 and P2 are not negligible. Once the bias voltage Vbias changes, the equivalent resistance of the calibration device 130 changes accordingly and thus results in luminance flickers on the display.
To settle the problems mentioned above, the drive circuit 100 could further comprise a stabilizing device 140. The stabilizing device 140 is coupled to the input end A of the calibration device 130 for suppressing surges in the bias voltage Vbias which occurs due to the switch of the output voltage Vout. For example, the stabilizing device 140 could comprise the voltage pulling down device 141, the voltage pulling up device 142 and the bias transmission device 143. FIG. 3A shows the timing diagram of the output voltage Vout, and FIG. 3B shows the timing diagram of the voltage provided by the stabilizing device 140 corresponding to the output voltage Vout. In FIG. 3B, the section 1, section 2 and section 3 are respectively caused by the voltage pulling down device 141, the bias transmission device 143 and the voltage pulling up device 142. In the section 1, the voltage is pulled down to the grounded voltage VGND to neutralize the surge P1 in FIG. 2; in section 2, the voltage is stabilized to be at the ideal level, the level of the bias voltage Vbias; and in section 3, the voltage is pulled up to the high voltage VH to neutralize the surge P2 in FIG. 2.
However, from FIG. 3B, the stabilizing device 140 does not perform very well in section 2 essentially because of the low charging speed of the bias transmission device 143. Therefore, if there is an apparatus for improving this performance, the brightness of the display will become more stable.